We have four - large greyhounds. Be prepared for the extra mess and hair - the house is given a hard time. There are times - in winter, say when the garden is muddy, that it feels like you are running to stand still WRT housework. It can get frustrating.
Not all your visitors may like dogs - mostly ours do, but I'm aware that not all family members are terribly keen. Similarly when doing visits - big difference when visiting with one or with three. With four, we find this has undoubtedly curtailed options -though if I'm honest, I don't really mind. 
Holiday accommodation - a surprising amount of "dog friendly" cottages etc will say things like "one well behaved dog only" or "up to two dogs". If you all want to holiday together, there are fewer places that will take three or more. They are out there but you will have to work harder to find them.
Transport - will you fit three in to whatever vehicle you currently use?
Dog/house sitting. It's very easy to find a person who'll pop in to let a dog out or even take one for a quick walk if you are having a day out somewhere, especially if it's a cute, elderly Cav. Much harder to find someone who's willing to do this for three (and virtually impossible for four!)
Food, vet's bills, finance - yes, it costs more. If your son is going to be paying for food, work out how this will pan out in practice - it may be easier and more cost effective for instance to buy large quantities of food and feed them together. Establish how you will receive his contribution to this.
Exercise - will he be exercising separately or together with you? This could work very well as long as he doesn't take the mick and expect you to do the lion's share of the exercising. Training - will you be expecting (and insisting) on him taking new dog to training classes, so minimum standard of behaviour? Untrained dog (especially if pup) not so good to be around and will have a negative impact on you and your other dogs.
Personally, I LOVE being a multi dog household - there are times when it's a bit chaotic, or when I just want the house to stay clean for longer than 30 seconds, but overall it's great. I've highlighted the negatives above, but you can't put a price on having a big pile of greyhounds on the bed with us in the mornings, or seeing their waggy tails and happy faces when it's time for a walk, or watching them playing together, or doing zoomies. 